In recent years, the demand for lead free in a piezoelectric material is increasing, and thus the researches on the preparation of the high-quality film using potassium sodium niobate [(K, Na)NbO3 (hereinafter also referred to as KNN)] based material and on the application of the film in the elements are actively developed. The KNN based material has a relatively high curie temperature and good piezoelectric properties in lead-free piezoelectric material, so it is expected to be used in piezoelectric film element.
As the application in the piezoelectric film element, for example, there are piezoelectric sensors which uses the piezoelectric effect that the force applied to the piezoelectric film is converted into a voltage. Particularly, a gyro sensor, a pressure sensor, a pulse wave sensor, a shock sensor, a microphone or the like can be listed. On the other hand, as the piezoelectric actuator where inverse piezoelectric effect is involved, a hard disk drive head, an ink jet head, or a loudspeaker, a buzzer, a resonator and the like where the inverse piezoelectric effect is involved similarly, wherein, the inverse piezoelectric effect is an effect by which the piezoelectric film will be deformed when a voltage is applied.
If the piezoelectric material is made as a film, the element may be downsized and can be used in more fields. In addition, as a plurality of elements can be prepared in a substrate in a lump, the productivity is increased. Further, there are lots of advantages in terms of performance such as the enhancement of the sensitivity when the piezoelectric material is prepared to be a sensor.
In order to obtain a good piezoelectric property, a piezoelectric film with a high quality is required. However, sometimes warp will occur in the piezoelectric film by the heating deposition because the difference of the coefficient of thermal expansion between the substrate and the piezoelectric film. The elements using such a piezoelectric film with warp may have a risk that their piezoelectric property will decrease when they are driven consecutively.
When facing the problem mentioned above, the technology described in Patent 1 took the difference of the coefficient of thermal expansion between the piezoelectric film made from KNN based material and the substrate into consideration, and prevented the decreasing of the piezoelectric property during consecutive driving by making the warp under the room temperature have a curvature radius of 10 m or above.